Electrophoretic deposition is a well known process useful to paint a variety of conductive substrates. Through electrophoretic deposition, automobile and truck bodies may be primed prior to topcoating. Electrophoretic deposition technology is discussed in a variety of publications including "Cathodic Electrodeposition", Journal of Coatings Technology, Volume 54, No. 688, pages 35-44 (May 1982). Briefly, a direct current is passed through an aqueous suspension of positively charged paint particles. Under the influence of the applied current, the charged paint particles migrate to and precipitate upon a conductive substrate of opposing charge. In cathodic and anodic electrophoretic painting, precipitation occurs on cathodic and anodic substrates, respectively.
Cathodic painting processes are now seemingly more popular than their anodic counterparts. In cathodic electrophoretic painting processes, paint particles are suspended in an aqueous carrier. Upon the passage of electrical current therethrough, water is electrolyzed. Hydroxyl ions formed at the cathode establish an alkaline diffusion layer contiguous therewith. The alkalinity of the diffusion layer is proportional to the cathode current density. Under the influence of the applied voltage, the positively charged paint particles electrophoretically migrate to the cathode and into the alkaline diffusion layer. If the cathode current density is sufficiently high, hydroxyl ions produced thereby raise the pH of the diffusion layer enough to ensure chemical reaction between the charged paint particles and the hydroxyl ions, whereby the former precipitate upon the cathodic substrate.
Cathodic electrophoretic painting apparatus used for large substrates such as truck bodies typically comprise an elongated, e.g., ca. 120 ft, tank for containing the paint bath. The substrate is submerged in the bath and conveyed along the length of the tank, through introductory, coating, and exit regions thereof. The introductory region, having no anodes, typically permits complete immersion of the substrate before its admission to the coating region. Passage through the introductory region lessens the condition known as hash marking, i.e., an uneven coating attributed to sudden exposure of the substrate to a high electrical potential difference.
The coating region of the tank, wherein painting occurs, typically comprises at least two distinct coating zones or electrode banks. Each bank comprises two opposing arrays of one or more anodes aligned along the longitudinal sides of the tank. Substrates are conveyed through the coating region between such opposing arrays. Each successive bank in the direction of substrate conveyance is maintained at a higher electrical potential than each preceding bank. For ease of handling and maintenance, such systems commonly use large, planar anodes, e.g., flat plate or box electrodes. Typical planar anodes for painting large objects, e.g., truck bodies, extend approximately three feet along the longitudinal sides of the tank in the direction of substrate conveyance and are typically approximately eight feet long, i.e., high. Planar anodes extend downward to at least the lower portion of the substrate, may be spaced approximately six inches apart within each coating zone and project a small distance, e.g. three inches, above the bath such that virtually the entire frontal face i.e., about 24 ft.sup.2, confronting the cathode, e.g. a car body, is effectively used.
Rather than using relatively few large planar anodes, some manufacturers have used a multitude of smaller anodes, e.g., two inch diameter tubes, continuously spaced from six to twenty-four inches apart along the entire length of the coating region, the degree of spacing increasing along the line of substrate conveyance.
Regardless of the type of anode utilized, the anodes and substrate are electrically coupled to a power source and to a ground by appropriate electrical conductors, e.g., bus bars and/or cables. As indicated, anode banks are maintained at successively greater electrical potentials along the line of substrate conveyance to compensate for the increased resistivity of the applied coating as it is deposited. This electrical potential gradient permits thicker, uniform paint deposition in a shorter tank than possible with single potential/zone systems.
While multi-zone cathodic painting represents a valuable coating technology, it heretofore has been encumbered by reverse current flow, the tendency of current to flow from a higher potential bank to the adjacent lower potential bank, rather than solely to the substrate. Such misguided current can lead to paint deposition on the lower potential bank anodes, and the general fouling thereof. Reverse current flow is particularly acute as a batch of substrates enters a substrate-free tank when the resistance between a higher potential bank and an adjacent lower potential bank is less than that between the higher potential bank and the entering substrates.
As will be discussed in conjunction with the appended Figures, it is known that reverse current flow may be reduced by electrically disconnecting, without physically removing, one or more of the lower potential anodes immediately adjacent the higher potential bank. Electrical disconnection precludes current flow from the disconnected lower potential anode(s) to other lower potential anodes through common conductors. Nonetheless, current can pass from the higher potential anode to the lower potential anode adjacent the disconnected anode either through the relatively resistive coating bath, i.e. and circumvent the disconnected anode, or be shunted through the electrically detached low resistance anode en route to the adjacent lower potential anode.
The Figures will further illustrate the known practice of providing a relatively large gap between adjacent banks of different electrical potential to more effectively curtail reverse current flow therebetween. A large gap may be formed by physically removing one or more anodes adjacent the high potential-low potential interface. The larger the gap provided, the higher the resistance between the adjacent banks. By providing a sufficiently large gap between adjacent banks of different potential, significant current flow therebetween, and concomitant lower potential anode fouling, can be reduced. At least one prominent electrophoretic paint system supplier advises that if adjacent banks differ in electrical potential by more than 75 volts, at least a two-cell length gap should be provided, a cell length being the anode dimension, e.g., the anode width, measured along the line of substrate conveyance plus the gap length between such anode and the next adjacent anode in the same bank. If adjacent banks differ in electrical potential by more than 100 volts, the supplier advises the provision of at least a three-cell length gap.
While providing a suitable gap between higher and lower potential zones reduces the magnitude of reverse current flow, the strategy suffers attendant disadvantages, especially when the recommended gaps are large. As the substrate passes the gap, the electrical current reaching it drops, resulting in slower paint deposition. The magnitude of current reduction and the length of time it exists are related to the gap length and the rate of substrate conveyance. Thus, providing a high resistance interzone gap requires either increasing the length of the tank and the coating region therein to ensure equivalent coating deposition at a comparable rate of substrate conveyance, or accepting a reduced effective immersion time of the substrate. The former leads to increased bath volume, tank floor space requirements, and actual substrate immersion time, which translates to correspondingly greater process costs. The latter leads to a non-optimal coating situation, i.e., fewer electrodes are available for electrophoretic deposition, resulting in thinner coatings.
Finally, it is known that the interzone gap may be reduced by interposing a diode between each of the anodes of the adjacent lower potential bank and their connected power source, averting current backflow therebetween. It is known that proper diode placement permits a one-cell interzone gap between adjacent banks. This technique nonetheless still involves an interzone gap devoid of anodes and associated electrochemical activity.
Manufacturers would find great advantage in an electrophoretic painting apparatus wherein paint deposition upon lower voltage large-faced anodes is minimized or precluded without interposing significant electrochemically inactive gaps between adjacent high and low potential anode banks. It would be desirable for such an apparatus to enjoy a coating region substantially filled with anodes, each contributing to the electrophoretic deposition process. Such continuous anode placement would allow a shorter tank, requiring less floor space and paint bath, and would permit shorter substrate residence times for the same coating thickness. Such an apparatus would afford significant financial savings.
Accordingly, it is an object of this invention to provide a multi-zone electrophoretic painting apparatus utilizing primarily large-faced planar electrodes, and having a coating region which is electrochemically active along substantially its entire length, while allowing negligible reverse current flow through the lower potential anodes.
This and other objects and advantages of the present invention will become more readily apparent to one skilled in the art through the description thereof which follows.